Thermal spraying, also knwon as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional theremal spray gun is used for the purpose of both heating and propelling the particles.
A thermal spray gun normally utilizes a combustion flame, a plasma flame, or an electrical arc to produce the heat for melting of the spray material It is recognized by those of skill in the art, however, that other heating means may be used as well, such as resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters.
The material may be fed into a heating zone in the form of powder or a rod or wire. In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame or the like, such as a combustion flame where it is melted or at least heat-softened and atomized, usually by compressed gas, and thence propelled in finely divided form onto the surface to be coated. In an arc wire spray gun two wires are melted in an electric arc struck between the wire ends, and the molten metal is atomized by compressed gas, usually air, and sprayed onto a workpiece to be coated. The rod or wire may be conventionally formed as by drawing, or may be formed by sintering together a powder, or by bonding together the powder by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the powder to be sprayed in finely divided form. In other forms the wire may have a coating sheath of one component and a core of the others, or may be made by twisting strands of the components.
European patent application, publication No. 0 118 307 (Tenkula), discloses a composite wire which can be used for arc gun spraying, comprising a sheath made from soft metal, such as soft alloyed steel, and a core consisting of metal powder or a mixture of a metal powder and "special carbides" and/or oxides. Examples in Tenkula of the special carbides are the carbides of chromium, tungsten, vanadium, titanium, niobium, silicon, molybdenum, boron and the like. Examples of the main composition of the core are taught to be C, Cr, Si, Mn, V, B, W, Mo, Nb, Ni, Co, Cu, Ti, Al and Fe. The grain size of the powder in the core is given as 20-300 microns.
Similarly, U.S. Pat. No. 3,332,752 (Batchelor et al) teaches the flame spraying of a filled tube containing powders of metals, alloys, oxides, carbides and lubricants, the powder size being greater than 600 mesh (20 microns).
Welding electrodes may be in composite form of a sheath and a powder core. The requirements for welding are somewhat different than for arc gun spraying in that the welding process melts and flows the material directly onto the workpiece. Thus such electrodes generally include a flux such as an active metal fluoride. U.S. Pat. No. 3,627,979 (Quaas) typifies the art on welding electrodes, disclosing a sheath of steel, nickel alloy or cobalt alloy filled with particles finer than 25 mesh (800 microns). The core may include refractory carbides of chromium, tungsten, titanium and vanadium.
Art such as described above involving carbide-containing composite wires for arc gun spraying, combustion flame spraying or welding is broadly directed to producing coatings that contain the carbide. To achieve this result the above references indicate that the core powder particles be relatively coarse, generally at least 20 microns where the lower limit is given. Such a lower limit on powder size is also generally indicated for any core powders including metals (e.g. Tenkula).
Certain compositions of composite materials not containing carbide particles nevertheless produce coatings that are quite hard and wear resistant. For example, U.S. Pat. No. 3,991,240 (Harrington et al) discloses a composite powder having a cast iron core clad with molybdenum and boron particles. Such a material is useful for such applications as piston ring coatings, being a potential improvement over the industry standard coating of thermal sprayed molybdenum. However, further improvement is desired to further extend the life of components in engines and the like.
Self-fluxing alloy powders of nickel and cobalt are quite common for hard facing coatings. They contain boron and silicon which act as fluxing agents during the coating operation and as hardening agents in the coating. Usually self-fluxing alloys are applied in two steps, namely thermal sprayed in the normal manner and then fused in situ with an oxyacetylene torch, induction coil, furnace or the like. The fluxing agents make the fusing step practical in open air. However, the alloys may also be thermal sprayed with a process such as plasma spraying without requiring the fusing step, but the coatings are not quite as dense or wear resistant. Generally self-fluxing alloy coatings are used for hard surfacing to provide wear resistance, particularly where a good surface finish is required since the fusing produces a coating having very low porosity. Typical self-fluxing alloy powder compositions are disclosed in U.S. Pat. Nos. 2,868,639 and No. 2,936,229.
In view of the foregoing an object of the present invention is to provide a novel composite wire useful in the arc gun spraying process.
Another object is to provide a novel wire for producing thermal sprayed coatings that have exceptional hardness and wear resistance.
Yet another object is to provide a novel wire for producing thermal sprayed coatings containing alloyed boron and carbon.
A further object is an improved arc gun spray process that incorporates the use of a novel composite wire.
These and still further objects will become apparent from the following description.